Influence of Defect Structures on Precipitation in Aluminium-Copper Alloys

Abstract

The effect of simultaneous deformation on the ageing kinetics of θ′ in aluminium-copper alloys has been followed by means of micro-indentation hardness measurements and transmission electron microscopy. The influence of deformation on precipitation was found to depend critically upon the initially formed defect structure. In dilute alloys (< 2 wt.-%Cu), deformation at fast and slow strain rates generated a subgrain structure, which resulted in slightly accelerated ageing kinetics. In the more concentrated alloys (2 and 4 wt.-% Cu) slow strain rates gave rise to extensive-arrays of helical dislocations within grains. These helices retarded ageing by absorbing vacancies and solute, thereby minimizing subsequent diffusion through the lattice. As the strain rate was increased, interactions of moving dislocations with helices led to elimination of the helices and the development of a subgrain structure. The substructure then approximated to that of the dilute aluminium-copper alloys, causing similar accelerated ageing kinetics. Using the models of Balluffi and Ruoff (J. Appl. Physics, 1963, 34, 1634) to describe vacancy/dislocation interactions within the dilute and concentrated aluminium-copper alloys it is possible to explain the observed accelerated and retarded ageing kinetics.